1. Field of the Invention
The present invention relates to a controller for controlling the rotational speed of a cam shaft in such a manner as to maintain the grinding speed constant during grinding on a cam grinding machine. More particularly, it relates to a controller for maintaining the grinding speed constant by applying to a servomotor, for cam shaft drive, a velocity command voltage whose level is changed step by step in correspondence to the rotational angular positions of the cam shaft.
2. Brief Description of the Prior Art
Generally, the grinding of cams on a cam shaft rotating at a constant speed causes the grinding speed to change in correspondence to the rotational angle of the shaft. For example, in the case of grinding a cam CM shown in FIG. 1, the grinding speeds at a base circle portion B and a top portion T of the cam CM are in proportion respectively to the lengthes of their radii, whilst the grinding speed at each side portion S rapidly changes due to the fact that a small change of the rotational angle effects a large travel of a grinding point P where a grinding wheel G contacts the cam CM.
FIG. 2 shows a relationship between the cam rotational angle and the grinding speed in the case of a constant cam rotational speed. It will be understood therefrom that the grinding speed is rapidly increased in the vicinity of a 140-degree angular position and of a 220-degree angular position where the grinding point P is on either of the side portions S. The rapid increase of the grinding speed creates excessive grinding resistance upon the wheel surface, and this results not only in uneven or local abrasion of the grinding wheel surface, but also a profile error in the final accuracy of the cam.
For the purpose of solving these drawbacks, there have recently been suggested various methods for controlling the cam grinding speed to be constant by changing a velocity command voltage applied to a cam drive servomotor, in correspondence to the rotational angular position of the cam CM. These methods are roughly grouped in three classes. In a first method the rocking angular position of a rocking table for carrying a cam shaft is detected so as to apply to a cam drive servomotor a velocity command voltage proportional to the detected rocking angular position. A second method involves storing in a semiconductor memory a plurality of velocity command values corresponding respectively to cam shaft rotational angular positions, and these values are successively read out in correspondence to the angular positions so as to be converted into respective analog signals for application to a cam drive servomotor. A third method uses a counter for detecting the actual rotational angular positions of a cam shaft by counting pulses which are generated in synchronism with the cam shaft rotation, and the counter count values serve to discriminate cam shaft rotational speed change positions with the result of changing step by step the velocity command voltage applied to a cam drive servomotor.
However, in the first method, it is impossible to compensate for an increase of the grinding speed which is caused by the travel of the grinding point P on each side portion S, since the method is intended to change the rotational speed of the cam shaft in proportion to a change in cam lift at the grinding point P. Since the second method stores the velocity command values in a semiconductor memory it is therefore difficult not only to set, but also to alter the stored velocity command values, and furthermore the provision of a digital to analog converter is also necessary. For these reasons, the third method is considered best in terms of both accuracy and manipulation. However, in the third method, the cam rotational speed change positions are detected in accordance with the count value indicated by the counter. Therefore, to detect whether the count value has been increased to any of preselected values or not, there are required a number of comparators corresponding to the number of speed change positions, thus complicating an electric control circuit for this method.